1. Field of the Invention
The present invention generally relates to masks for printing solder paste and methods of soldering electronic parts using the mask, and more particularly to a mask which is used to print a solder paste on a printed wiring board and a method of soldering electronic parts using such a mask.
2. Description of the Related Art
Presently, solder is used as the bonding material when bonding the electronic part to the printed wiring board because of the many advantageous features of the solder. As methods of supplying the solder to the printed wiring board, there is a first method which employs screen (or stencil) printing of the solder paste, a second method which generates a solder film by plating on a foot print (pad), and a third method which uses chemical reaction.
The first method is inexpensive and easy, but it is difficult to apply the screen printing to fine pads. On the other hand, the second method involves a complex process and is expensive, and further, it takes time to obtain the desired film thickness. Moreover, the film thickness is considerably inconsistent on a large board, and the plated film thickness is normally on the order of 10 .mu.m which is small. As for the third method, special equipment (technique) and materials are required, and involves a complex process and is expensive as in the case of the second method.
Accordingly, the second and third methods are, in most cases, applied to fine pads, and the screen printing or the like is made independently with respect to normal pads. As a result, the method of supplying the solder requires two independent processes which are troublesome to carry out, and there is a demand to improve the first method.
FIG. 1 is a diagram for explaining an example of a surface mounting of electronic parts on a board. In FIG. 1, (A) shows a plan view and (B) shows a cross section along a line A--A in (A). In FIG. 1, there are shown a circuit board (printed wiring board) 1, fine pads 2, normal pads 3, a fine pitch part (semiconductor part) 5 such as a quad flat package (QFP), and normal pitch parts (circuit parts) 6 such as resistors and capacitors.
Because of the reduced size and narrow pitches of the recent electronic parts, a solder film in the form of a thin paste is formed on the fine pads 2 and the normal pads 3 on the circuit board 1. Then, the fine pitch part 5 and the normal pitch parts (chip parts) 6 are mounted by reflow soldering. Hence, it is possible to realize a high density surface mounting.
For example, leads (not shown) of the QFP 5 have a width of 150 .mu.m and are arranged at a pitch of approximately 0.3 mm. The rectangular (box-shaped) fine pads 2 having a length of 2 mm and a width of 150 .mu.m are also arranged at a pitch of 0.3 mm so as to surround the QFP 5 located at the center. On the other hand, the normal pads 3 have a square shape having a side of approximately 1.5 mm and are arranged at a sufficiently large pitch.
FIG.2 is a diagram for explaining a conventional screen printing. In FIG.2, (A) shows a plan view, and (B) shows a cross section along a line A--A in (A). In FIG.2, those parts which are the same as those corresponding parts in FIG.1 are designated by the same reference numbers. In FIG.2, there are additionally shown a squeegee 7, a solder paste 8, a metal mask (metal screen) 10, fine mask holes 12, and normal mask holes 13. The rectangular fine pads 2 having a length L=2 mm and a width W=150 .mu.m are arranged at a pitch P=0.3 mm on the circuit board 1, and the normal pads 3 which are sufficiently large compared to the fine pads 2 are arranged on the periphery of the fine pads 2.
The metal mask 10 has a plate thickness t of approximately 150 to 200 .mu.m. The fine mask holes 12 having a shape approximately the same as that of the fine pads 2 are formed in the metal mask 10 at positions corresponding to the fine pads 2, and the normal mask holes 13 having a shape approximately the same as that of the normal pads 3 are formed in the metal mask 10 at positions corresponding to the normal pads 3. The aperture size (1, w) of the mask holes 12 and 13 is normally selected to a shape which is reduced by several tens of .mu.m from the foot print size (L, W).
The metal mask 10 is aligned to and placed on the circuit board 1, and the solder paste 8 is coated on the entire surface of the metal mask 10. Then, the squeegee 7 is moved while pushing against the metal mask 10 so as to fill the solder paste 8 into the mask holes 12 and 13, and the metal mask 10 is thereafter removed. As a result, a solder paste film having the same thickness as the plate thickness t of the metal mask 10 is formed above the pads 2 and 3, and the amount of supplied solder is determined by the plate thickness t of the metal mask 10 and the mask aperture area (1.times.w).
When making the screen printing with the solder paste using the aperture shape of the metal mask 10 described above, it is regarded that the optimum film thickness of the normal pad 3 is approximately 150 to 200 .mu.m and the optimum film thickness of the fine pad 2 is approximately 100 .mu.m.
However, if the conventional metal mask 10 is used, the fine pads 2 which are actually formed also have a film thickness of approximately 150 to 200 .mu.m. For this reason, there is a problem in that the thick fine pads 2 may generate a solder ball or cause a bridge which would cause a short-circuit. In other words, the conventional method supplies too much solder.
Accordingly, the plate thickness t of the entire metal mask 10 has conventionally been made small. However, when the plate thickness t is made small, it is possible to make the film thickness of the fine pad 2 an optimum value but the film thickness of the normal pad 3 becomes insufficient.
In addition, the aperture width w of the fine mask hole 12 has conventionally been reduced further. But as described above, the plate thickness t of the metal mask 10 is 150 to 200 .mu.m and the aperture width w of the fine mask hole 12 is 150 .mu.m or less. For this reason, the aspect ratio (w/t) at the cross section of the fine mask hole 2 is 1 or less.
In general, if the aspect ratio becomes 1 or less, it is known that the aperture size at the middle part of the side wall of the fine mask hole 2 becomes smaller than the aperture size at both the front and back surfaces of the metal mask 10 when carrying out the etching process with respect to the metal mask 10. For this reason, the solder film formed on the fine pad 2 becomes narrower than the desired value if such a metal mask 10 is used, and there is a problem in that the reliability of the reflow soldering becomes poor.
In addition, if the aspect ratio becomes 1 or less, the friction between the solder paste and the sidewall of the fine mask hole 12 becomes large when removing the metal mask 10 after the screen printing. As a result, the solder paste does not escape from the fine mask hole 12 in a satisfactory manner, and there are problems in that the printing becomes thin, the supply of the solder paste is insufficient and the like. Furthermore, there is also a problem in that precision forming the fine mask holes 12 having the aspect ratio of 1 or less is extremely difficult.
If only the improvement of the aspect ratio is considered, it is possible to make the aspect ratio greater than 1 by making the fine mask width w greater than the fine pad width W. But in this case, an excess amount of solder paste would be supplied, and the possibility of forming a solder ball or bridge would become high.
Conventionally, the fine mask holes 12 have the rectangular shape to match the rectangular shape of the fine pads 2. Hence, there has been a problem in that the printing characteristic of the solder paste greatly differs depending on the moving direction of the squeegee 7. Consequently, controlling the supply of solder paste, such as changing the mask aperture shape (printed board) depending on the moving direction of the squeegee 7, is extremely delicate and difficult.
FIG. 3 is a flow chart for explaining the conventional soldering method. In a step S1, the above described metal mask 10 is used to do the screen printing of the solder paste on each pad of the circuit board. In a step S2, the normal pitch parts and the fine pitch parts are mounted on the pads in one process, and a reflow soldering is done in a step S3. That is, a process of preheating to a temperature of 120.degree. to 160.degree. C. for 20 to 30 seconds is performed in a step S24, and a main process of heating to a peak temperature of 230.degree. C. is performed in a step S25.
However, when the screen printing is made by the conventional metal mask 10, an excess amount of the solder paste is supplied on the fine pads, and the flux component within the solder paste is melted by the preheating process prior to the main heating process. As a result, the excess solder particles are pushed to the outside by the weight of the fine pitch parts, and the solder ball and bridge phenomena frequently occur.
Therefore, it has conventionally been difficult to supply an appropriate amount of the solder paste on both the fine pads and the normal pads of the circuit board in one screen printing process using the conventional mask.
In addition, the solder ball and bridge phenomena frequently occur when employing the conventional soldering method to form the fine pads in particular.